High pressure sodium metal vapor (HPS) lamps are widely used today for outdoor lighting, because of their high luminous efficiency. Although they produce a pink-to-yellow light output, their low power consumption and long life make them particularly attractive for street lighting where lamps of 150 to 1000 W size are used. Recently high pressure sodium lamps in sizes as small as 35 W have been constructed so that this high efficiency lighting device can be used in a variety of applications where a lesser amount of light is needed. Such small HPS lamps can often replace incandescent lamps of 100 to 150 W size with considerable power savings.
The typical HPS lamp is contructed from a cylindrical tube of translucent polycrystalline alumina with tungsten electrodes sealed in at both ends. Within this arc tube is an electrical discharge in a mixture containing mercury, sodium vapor and a noble gas, usually xenon. Current is conducted to the electrodes via a feedthrough assembly, consisting of a niobium metal rod or tube sealed to the alumina arc tube, with a meltable frit composition based on calcium aluminate. Both the niobium and the frit are chosen to match in thermal expansion that of the alumina arc tube. The complete arc tube and electrode assembly is mounted within a glass outer envelope which is evacuated to high vacuum to avoid oxidation of the exposed metal parts that reach several hundred degrees Celsius when the lamp is operating normally. The metal (Nb) in the arc tube feedthrough oxidizes readily and, therefore, air or oxygen must be removed from the outer jacket. Usually it is evacuated to prevent metal oxidation, assist in maintaining a sufficiently high arc tube end temperature, and at the same time improve lamp efficacy by insulating the arc tube from the surroundings, thus reducing thermal conduction losses.
The arc tube is assembled by placing a first feedthrough and electrode assembly on top of a polycrystalline alumina tube with a preformed ring of frit compostion in between, and heating in an atmosphere of at least 200 torr of argon, the top half of the tube from 1400.degree. to 1500.degree. C. The frit composition melts and the feedthrough assembly settles in place with the melted frit filling the space between it and the alumina tube. After cooling, the partial assembly is inverted, an amalgam pellet containing sodium and mercury is filled into the open end of the arc tube in a dry environment, and a second feedthrough and electrode assembly is placed on top with a second preformed ring of frit composition in between. The second seal is made by heating the top half of the tube from 1400.degree. to 1500.degree. C. in an atmosphere of xenon at about 20 torr pressure. When the frit melts and flows, the feedthrough assembly settles in place, trapping a predetermined amount of xenon and the amalgam pellet with the arc tube. After cooling the arc tube assembly is ready for mounting inside the outer glass lamp envelope.
When the above production procedure is applied to HPS lamps of low wattage rating, a problem arises because of the small sixe of the polycrystalline alumina arc tube. While making the second seal at about 1400.degree. to 1500.degree. C. at the top of the arc tube, the bottom end of the arc tube where the amalgam pellet is resting rises in temperature to about a few hundred degrees Centigrade, and the mercury begins to volatilize in the low xenon pressure environment. The mercury vapor displaces the xenon and often prevents the feedthrough and electrode assembly from settling in place as the frit melts.
Although the lower portion of the arc tube is supported in a conduction-cooled metallic heat sink while the upper portion is radiantly heated, the high temperature differential between the top end being sealed and the lower end with the amalgam is difficult to maintain when the arc tube length is as short as 3.8 cm (as with the 35 W design), or even shorter.
HPS lamps are usually filled with an excess of amalgam to compensate for Na loss during the life of the lamps (16,000 to 24,000 h) known to occur in a small degree by diffusion through the polycrystalline ceramic arc tube and mainly through defects of the polyphase ceramic sealing frit at the end of the arc tube, which is the cold spot and location of the excess sodium fill behind the electrodes (see, FIG. 1). Loss of sodium causes a shift in the equilibrium partial vapor pressure ratio of Na and Hg, which is a critical lamp design parameter, and has to be adjusted to maintain a sodium D-line peak separation of about 8.5 nm for optimum efficacy of the lamp.
The stabilization of mercury by compound formation and its subsequent release by dissociation has been described by Keller in U.S. Pat. No. 3,318,649, and by Della Porta, et al. in U.S. Pat. Nos. 3,657,589 and 3,733,194, which are all incorporated herein by reference. In each case, an intermetallic compound is formed between a highly electropositive metal and the weakly electropositive element mercury and subsequently dissociated to release the mercury. The most satisfactory compounds would appear to be Ti.sub.3 Hg and Zr.sub.3 Hg which, according to the latter patents, resist dissociation up to about 550.degree. C. and are recommended for uses in dosing fluorescent lamps.